The object of the present invention was to make a vaccination virus. This objective has been fulfilled with the segmented virus constructed as described herein.
The genome of influenza A viruses consists of 8 different single-stranded viral RNA (vRNA) molecules of negative polarity, which have in common 5xe2x80x2 and 3xe2x80x2 terminal sequences largely complementary to each other. These conserved segments 13 and 12 nucleotides in length are known to form double-stranded RNA panhandle structures (Hsu et al., 1987; Fodor et al., 1993) which have been analysed in more detail recently in vitro using internally deleted model RNAs (Baudin et al., 1994; Tiley et al., 1994). In the virion the panhandle ends of all RNA segments are found in specific binding to viral RNA polymerase complexes, while the remaining internal segments stay single-stranded with viral nucleoprotein (NP) in cooperative binding (Compans et al., 1972; Honda et al., 1988; Martin et al., 1992). Upon infection these viral RNPs initially serve as templates for the synthesis of viral mRNAs by a specific cap-snatching mechanism (Plotch et al., 1979; Braam et al., 1983), and later on will direct synthesis of full-length complementary RNAs (cRNAs), probably dependent on the absence or presence of newly synthesized NP protein (Shapiro and Krug, 1988). The plus-strand cRNAs are then used as templates for progeny vRNA synthesis.
The viral RNA polymerase complex consisting of proteins PB1, PB2, and PA is involved in all three different modes of RNA synthesis during the viral replication cycle, following its specific binding to the terminal panhandle segments of both vRNAs and cRNAs. Sequence comparison reveals that the vRNA and cRNA termini have similar, but not identical sequences. For that reason vRNA and cRNA recognition may be distinguished because of these structural alterations allowing for asymmetries in initiation of plus and minus strand RNA synthesis, and possibly in viral RNP packaging, which has also been suggested to be controlled by the panhandle RNA sequence (Hsu et al., 1987).
Recently, we reported on an in vivo system for the introduction of specific mutations into the genome of influenza viruses: viral cDNA has been inserted in antisense orientation between mouse rDNA promoter and terminator sequences.
This has been derived from in vitro transcription experiments based on nuclear extracts from Ehrlich ascites cells, which resulted in transcripts exactly resembling influenza vRNA. For a series of in vivo studies, the viral coding sequence was replaced by the coding sequence for chloramphenicol-acetyltransferase (CAT), however, with both influenza terminal non-coding sequences being retained exactly on the resulting vRNA transcripts. After transfection of this recombinant DNA template into mouse cells followed by influenza virus infection, CAT activity was detectable. Transfer of supernatants to different cells demonstrated that CAT-vRNAs transcribed in vivo by cellular RNA polymerase I were not only transcribed by viral RNA polymerase into plus-strand mRNA and translated into CAT protein, but also were replicated and packaged into infectious progeny virus particles (Zobel et al., 1993; Neumann et al., 1994).
We have used this system for a stepwise introduction of single and multiple mutations into the conserved panhandle RNA sequence, thereby effectively converting the HA-vRNA promoter sequence into an HA-cRNA promoter sequence and vice versa. For these series of constructs CAT activities have been measured both in primarily transfected and infected B82 cells and, after passaging of B82 supernatants, in secondarily infected MDCK cells. From the results obtained we propose a model for the terminal RNA sequence as being recognized RNA polymerase in consecutive steps of different structure when used as a template for initiation of viral mRNA synthesis.